So let me get this straight: he has services because he has learning challenges, and those services (like being tested in a separate room) have allowed him to test at grade level so... the result is to remove the services that got him there?
Have you asked them about this catch 22? I would be interested in their response. My advice is to seek out a private assessment, yes it's expensive if you have no coverage, but it IS WORTH IT. Not only to help ensure he keeps his services, but because he, as a human being, deserves to know who he is. If Autism is a part of him, he is entitled to know it, and to know there is a big, welcoming community of others who are like him.
Whatever you do, do NOT allow his services to be yanked. Do whatever you need to do to keep things status quo, if status quo is working. Fifteen is such a pivotal age - he needs stability to get him through high school, not a radical change in the educational approach his school takes toward him.